1. Field of the Invention
The present invention generally relates to the field of electro-optical liquid crystal devices, and more particularly to a high contrast liquid crystal display panel.
2. Description of the Related Art
Liquid crystal displays are widely used in industrial, consumer, aerospace, and other applications. Such devices generally include a liquid material which exhibits bire-fringence and dielectric anisotropy, and whose alignment direction is switchable in response to an applied electric field. Of particular interest are liquid crystal materials which are switchable between light scattering and transparent states with an applied electric field.
Polymer dispersed liquid crystal display (PDLC) cells such as disclosed in U.S. Pat. No. 4,688,900, entitled "LIGHT MODULATING MATERIAL COMPRISING A LIQUID CRYSTAL DISPERSION IN A PLASTIC MATRIX", issued Aug. 25, 1987, to Doane et al, and U.S. Pat. No. 4,728,547, entitled "LIQUID CRYSTAL DROPLETS DISPERSED IN THIN FILMS OF UV-CURABLE POLYMERS", issued Mar. 1, 1988, to Vaz et al, have been recently developed. These cells include droplets of a liquid crystal material dispersed in a light transmitting matrix in the form of a flexible plastic sheet or film, such as an ultraviolet cured epoxy or thiol-ene monomer. The liquid crystal directors in the droplets are randomly oriented in the absence of an applied electric field, whereby the index of refraction of the droplets varies locally and the droplets scatter light incident thereon from any angle. Application of a sufficiently high electric field by means of indium tin oxide (ITO) film electrodes or other appropriate means causes alignment of the liquid crystal directors parallel to the applied field when the liquid crystal has a positive dielectric anistropy. The materials are selected such that the index of refraction of the liquid crystal material in the field-aligned state is equal to the index of refraction of the isotropic plastic matrix material for light at normal (perpendicular) incidence to the PDLC cell. Therefore, the cell appears optically transparent in the presence of an applied electric field when it is viewed at normal incidence.
A display panel is constructed by providing a liquid crystal cell having segments or other picture elements which can be collectively or individually switched on and off to form a desired pattern. Applications of liquid crystal devices cover a wide range from small consumer timepieces to relatively large dashboard and heads-up displays for automotive and other vehicle control panels.
A major problem which has persisted in many liquid crystal displays is low optical contrast between bright and dark image areas. Especially in the presence of sunlight and other high ambient light levels, conventional liquid crystal displays appear washed-out and are difficult or impossible to read.
Liquid crystal displays are conventionally illuminated from the back in a transmission mode or from the front in a reflection mode. A general description of these standard modes of operation is found in "Liquid Crystals and Their Applications", by T. Kallard, Optosonic Press, New York, N.Y., 1970. In PDLC cells, most of the light is forward scattered in a relatively small angle cone. When viewing just the scattered light from rear-illuminated PDLC cells, it has been shown, as described in a paper by A. M. Lackner, et al, Proc. SPIE Vol 958, Automotive Displays and Industrial Illumination, p.73-79, 1988, that the maximum contrast ratio is exhibited when the illumination light was about 30.degree. off normal incidence. Illumination of a PDLC cell through an edge thereof, which is perpendicular to the orientation of the liquid crystal directors in the field aligned state, would have been expected to produce a display with both on and off segments appearing bright due to scattering by the liquid crystal droplets.